Testing of an automatic outdoor gamma ambient dose-rate surveillance system in Tokyo and its calibration using measured deposition after the Fukushima nuclear accident.

An in-situ fixed point radioactivity surveillance network has been developed at the Radiation Protection Bureau, Health Canada. The network consists of a number of spectrometric NaI(Tl) detectors measuring, in real-time, ambient gamma dose-rate. The present paper describes the gamma dose-rate monitoring by one detector installed at the Canadian embassy in Tokyo during the Fukushima nuclear accident. Soil samples were collected for the measurement of fallout fission products inventories at each location where the NaI(Tl) detector was installed. The gamma-ray attenuation by the soil matrix was estimated by the information on the depth distribution of (137)Cs activities. The study demonstrated that the gamma dose-rates measured by the field NaI(Tl) spectrometric method agreed well with the laboratory results estimated by the inventories of fallout fission products deposited in the soil and the vertical distribution of (137)Cs in the soil.